Fluidized bed development apparatus

ABSTRACT

An apparatus for developing a latent electrostatic image wherein the developer material is circulated past the image bearing surface in a fluidized state. The fluidization of the developer material is achieved by the use of a vibrated bed mechanism which agitates the developer into a fluidized state in the development zone. The mechanical agitation or vibration of the fluidizing mechanism is effected through the use of two counter rotating weights which achieve a controlled degree of vibration with a minimum force and vibration being imparted to the xerographic machine in which the development device is utilized.

United States Patent 72] Inventors John S. Bernhard Pittsford; Alan F. McCarroll, Rochester, both of NY. [21] Appl. No. 78 [22] Filed Jan. 2, 1970 [45] Patented Nov. 30,1971 [73] Assignee Xerox Corporation Rochester, N.Y.

[54] FLUIDIZED BED DEVELOPMENT APPARATUS 14 Claims, 6 Drawing Figs. [52] U.S.Cl 118/637 [51] Int. Cl B051) 5/00 [50] Field of Search 118/308, 309, 312, 621, 637 [5 6] References Cited UNITED STATES PATENTS 3,372,675 3/1968 Tressel 118/637 3,393,663 7/1968 Donalies 3,536,043 10/1970 Eppeetal.....

Primary Examiner-Henry S. Jaudon Attorneys-James .l. Ralabate, Donald F. Daley and Terry J.

Anderson ABSTRACT: An apparatus for developing a latent electrostatic image wherein the developer material is circulated past the image bearing surface in a fluidized state. The fluidization of the developer material is achieved by the use of a vibrated bed mechanism which agitates the developer into a fluidized state in the development zone. The mechanical agitation or vibration of the fluidizing mechanism is effected through the use of two counter rotating weights which achieve a controlled degree of vibration with a minimum force and vibration being imparted to the xerographic machine in which the development device is utilized.

PATENTED 30 3. 623 .454

SHEET 1 BF 2 mvsmorzs JOHN s. BERNHARD BY ALAN F. McCARROLL ATTO EY 1 FLUIDIZED BED DEVELOPMENT APPARATUS BACKGROUND OF THE INVENTION This invention relates in general to developing a latent electrostatic image and in particular to an apparatus for circulating developer material in a fluidized state past a photosensitive surface supporting a latent electrostatic image to be developed.

More specifically this invention relates to a development apparatus wherein a housing or bed is agitated or vibrated to fluidize developer material thereon. The bed supporting the developer material is vibrated by counter rotating weights which achieve a highly controlled level of vibration in the bed with a minimum of vibration and force being transmitted to the support frame supporting the development device.

In the process of xerography, a xerographic plate comprising a layer of photoconductive material and a conductive backing is given a uniform electric charge on the surface and then is exposed to the subject matter to be reproduced by various projection techniques. Thisexposure discharges the plate in accordance with the light intensity reaching it thereby creating a latent electrostatic image on or in the plate. Development of the image is effected by developers which may comprise in general a mixture of suitable, pigmented or dyed, resin powder, hereinafter referred to as toner, which is xerographic image may be transferred to a support material to which it may be fixed by any suitable means such as heat fusing.

Various development devices have been utilized in xerography to develop a latent electrostatic image formed on a photosensitive surface. One well-known technique ofdevelopment is disclosed in US. Pat. No. 2,573,881 to Walkup et al., wherein toner carried by carrier particles is rolled or cascaded over the latent electrostatic image bearing surface. The carrier and toner materials are selected so so that a triboelectric atcomponents causing toner to be stripped off the carrier and electrostatically bonded to the charged image to effect development thereof.

Several difficulties are associated with the aforementioned cascade method of development. Cascade development, for

edges are comparatively Cascade development of developer after repeated development. If intermixing does not occur, a defect in development referred to as image strialocalized carrier material becoming depleted of toner powder. NOrmally, toner depletion occurs in cascade development when the carrier and toner materials move past the image in one direction across the photosensitive One attempt in the prior art to overcome some of the aforementioned problems and others presented by the cascade technique of development has been by effecting turbulence of the developer material while it is in the vicinity of the image bearing surface to achieve a fluidization of the developer. An example of such a fluidizing of developer material is disclosed in the US. Pat. No. 3,396,700 to Donalies wherein a two-component developer is agitated into a fluidized state in the development zone. The fluidization of developer is achieved by the imparting of vibration to a plate-supporting developer in the vicinity of the support surface. The agitation of the developer results in the material attaining a fluidized state to achieve uniform contact of developer with the photoconductor surface. Accordingly, the developer material is presented to the image area with a consistent density to produce better quality development of the latent image. Further, the fluidization of developer results in retoning of the carrier particles in contact with the image by eflecting transfer of toner from the underlayers of the developer to the toner depleted carrier in contact with the image. Also, the turbulence of the material causes some of the carrier particles not in direct contact with the photosensitive surface to be interchanged with the carrier particles in contact therewith to further insure sufficiently tonerized carriers are adjacent the image.

Fluidized developer also produces increased development contact with the latent image and alleviates streaking thereof to enhance development as compared to the cascade method. However, even though prior art fluidized bed devices improve development as compared to cascading technique of development, several problems are encountered when utilizing such a process. One deficiency related to the use of a fluidizing development device is caused from the inherent mechanical motion needed to agitate the developer material. Such a mechanical agitation of the developer has in the prior art imparted undesired vibrations to the support frame of the xerographic machine which may result in interference with the production of a duplicate copy. For example, the imposition of vibration from a fluidized bed to a xerographic machine decreases the resolution of the image formed on the drum surface since the vibration of the machine causes movement of the optical system and the drum to cause an image of poor ofthe machine upon which it is mounted.

Further, the prior art fluidizing development devices have utilized such means as a pneumatic mechanism or an electric motor to impart vibrations to a developer bed or housing to agitate the material into a fluidized state. However, such devices must be mounted on the support structure to induce vibration and therefore transmit the aforementioned undesired forces to the xerographic frame. Further, pneumatic vibrations are also available for use in vibrating a bed, but it is found that the use of high-pressure air in a system using xerographic developers is undesirable since the air tends to divert or otherwise disturb the small xerographic particles. Also since the housing or bed supporting developer in the development zone is relatively large, it is necessary to utilize a significantly powerful electric or pneumatic motor to oscillate the bed developer which is uneconomical in achieving desired fluidization. Therefore, it is advantageous in xerographic development to provide an apparatus for economically oscillating a fluidizing bed or housing to effect a selected frequency, direction, and amplitude of vibration thereof.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to develop a latent electrostatic image with an improved apparatus.

Another object of this invention is to improve the fluidizing of development material adjacent an image bearing surface.

A further object of this invention is to minimize vibrations being imparted to a xerographic machine from a vibrating housing of a development apparatus.

Still another object of this invention is to improve the selectivity of the amplitude and frequency of the vibration of a housing effecting fluidization ofdeveloper material.

These and other objects are attained in accordance with the present invention wherein there is provided an effective development device which overcomes many of the aforementioned difficulties presented by the prior art fluidized development systems. The invention comprises a fluidizing development device wherein a housing or bed is given a controlled vibratory movement to effect a fluidization of the development material lying thereon. The vibration inducing mechanism of the present invention includes two counter rotating weights mounted in such a manner to impart a minimum of vibration to the support structure in the machine on which the development device is mounted.

The vibration inducing apparatus of the present invention provides a high degree of vibration to a bed or housing of developer material ofa development device through the use of relatively small counter rotating weights. In one embodiment of the invention the bed of housing of developer material is supported on resilient means wherein substantially no vibration is imparted to the xerographic apparatus to impair reproduction quality. The two weights inducing vibration to the bed are rotated in opposite directions at the same angular velocity wherein the ratio of mass times radius of rotation of the weights are equal. With such a relationship a force to induce vibration is applied to the bed in a direction depending on the phase angle between the instantaneous locations of the weights whereupon the centrifugal force components of the rotating weights substantially cancel each other out in a direction normal to the direction of application of the vibratory force. As a result, a vibration inducing force is applied to the bed in the form of a sine function. An especially advantageous location for mounting the counter rotating weights of the aforementioned resiliently mounted embodiment of the invention is at the center ofgravity of the development device, because coupling effects caused by the weights are eliminated. Further, the use of the two counter rotating weights has been found to produce a highly controlled and significant level of vibration being induced to the housing to create a fluidization of developer material in a manner with a minimum of vibration being transmitted to the support frame. Further, since the rotating weights are small in relation to the amount of vibration imposed on the developer housing, an economical manner of creating fluidization of developer material is achieved through the utilization of the development apparatus ofthe present invention.

In another embodiment of the invention, the fluidized bed of housing is mounted as a pendulum. Therefore, by applying the force created by the two counter rotating weights at the center of percussion of the vibrating housing assembly, the force transmitted to the machine frame is also minimized so that the resolution capability of the xerographic machine is not affected in a manner previously described. Further, the oscillation applied by the counter rotating weights will produce a readily adjustable frequency and amplitude according to the value of the weights, the distance from the center around which the weights are rotating and the angular velocity of the weights. Therefore, the development apparatus of the present invention economically achieves a controlled level of fluidization of developer in a housing with minimal transmittal of forces to the xerographic machine.

Further objects of this invention, together with additional thereto and advantages accruing BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic illustration ofa drum-type xerographic FIG. 2 is a schematic illustration of the motion of the vibration inducing weights ofthe present invention;

FIG. 3 is a front schematic illustration of one embodiment of the fluidizing development device ofthe present invention;

FIG. 4 is an end schematic illustration of the embodiment of Fig. 3 of the present invention;

FIG. 5 is a schematic illustration of vibration inducing apparatus utilized in the embodiment of Fig. 3 of the present invention;

FIG. 6 is a schematic illustration of another embodiment of the fluidizing development device of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 there is shown a schematic view of a drum-type xerographic machine utilizing the development device of the present invention. The central element of the machine is a drum 1 mounted by suitable means and drivable in a conventional direction by a motor M. The drum 1 comprises an outer surface having a layer of photoconductive insu' lating material such as vitreous selenium or any other suitable surface. A uniform electrical charge is placed on the photoconductive surface of the drum 1 by means of a conventional corona charging device 2. The uniformly charged surface is then moved to exposure means 3 which may be any well-known device which will expose the charged surface to copy to be reproduced to form a latent electrostatic image of the copy on the photosensitive drum surface.

Following the formation of the latent electrostatic image of the copy, the image on the drum will move to development device 10 according to the present invention (which will he described in detail later) to bring the latent electrostatic image in contact with development material comprising carrier particles and electroscopic toner for development thereof. After development, material comprising carrier particles and electroscopic toner for development thereof. After development. the visible image moves to a transfer means 4 and is transferred from the drum to a web of paper 5 or the like which is positioned into contact with the drum by rollers 6. A second corona charging device 7 applies a charge to the back of web 5 to facilitate transfer of the toner powder in image form. The toner image on the web then moves past the heating element 8 which permanently affixes the toner to the paper web to form a duplicate of the original copy. A rotating brush 9 contacts the drum surface after it moves past the transfer prior to the subsequent reproduction cycle. It is clear that other modes of charging, exposing, transferring, or fusing may be utilized in conjunction with the present invention.

Referring now to FIG. 2, there is shown a diagrammatic illustration of the rotation ofthe counter rotating weights for inducing vibration of developer housing according to the present invention. The counter rotating weights comprise a suitable weight M rotated or orbited around a center of rotation in a first direction at a radius R,. A second weight M is rotated or orbited in the opposite direction at radius R whereby the weights are moved at the same angular velocity. The two counter rotating weights are selected to create the relationship that M, R,=M R Therefore, a resultant force is applied to the bed as a sine function in a direction depending on the phase angle between the two weights because the centrifugal force component of one of the weights along an axis normal to the direction of the resultant force cancels out the centrifugal force component of the second weight. The counter rotation of the two weights as illustrated in Fig. 2 has been found to create a high degree of leverage to produce a significant amount of vibration upon a developer bed or housing and achieve an economical and efficient degree of fluidization ofdeveloper material.

Referring now to FIGS. 3 and 4 there is illustrated an embodiment of the fluidized development apparatus 10 shown in Fig. 1 according to the present invention. In particular, in Fig. 3 there is shown a housing 11 adapted to support and fluidize a supply of developer material including carrier and toner powder to effect development of the latent electrostatic image created on the drum 1. The housing or bed 11 is supported adjacent the bottom of the drum by suitable resilient mounting means 12 such as, for example, leaf springs or the like. The edge of the walls 13 adjacent the drum surface are provided with a suitable seal means (not shown) to prevent loss of developer therefrom as the member is vibrated to create a fluidized bed of developer therein. Vibration created by the counterrotating weight mechanism 20 (to be described in detail later) causes the carrier and toner to be agitated to such a state to achieve a fluidization of the material forefficient development. The housing 11 may include internally mounted wires, screens, vanes or the like (not shown) to increase the fluidization of the developer in the housing, if desired. The vibration inducing apparatus 20 according to the present invention is suitably mounted within the housing or bed 11 supporting the developer to impose a desired level and form of oscillation on the housing. Although the vibration inducing apparatus 20 may be mounted at any location internally or externally of the housing, the vibration inducing apparatus 20 is illustrated as mounted by a suitable support at approximately the center of gravity of the development device to eliminate or minimize coupling efiects produced by the rotating weights. Therefore, by mounting the vibration inducing apparatus at the center of gravity of the development assembly 10 wherein the housing 11 is mounted on resilient means 12, practically no force or vibration is transmitted to the xerographic machine for interference with the operation thereof. The axis around which the weights of the vibration inducing apparatus rotate is illustrated as being perpendicular to the axis of rotation of the drum, but it should be apparent that the weights can be orbited around any axis depending on the desired oscillating motioii, frequency, amplitude and the like ofthe bed 11.

Referring now to FIG. 5 there is illustra ed an example of the vibration inducing mechanism 20 of Fig. 3. Although it is within the scope of the present invention to utilize any suitable mechanism to produce the motion of the counter rotating weights, for convenience of illustration of the apparatus of Fig. 5 is described as a form of the vibration means. The vibration inducing apparatus 20 of FIG. 5 includes a housing 21 suitably mounted at the center of gravity of the development housing or bed 11 wherein the housing 21 isolates the elements of the vibration inducing apparatus from the developer in the development housing. Two eccentric weights 22 and 23 are mounted for rotation within the vibration inducing housing to impart vibration on the development housing based on the aforementioned principle of counter rotating two masses.

The first weight 22 is removably mounted on an outer surface of an outer drum 24 by means of suitable bolt means 25. An input shaft 26 rigidly supports the outer drum whereby the input shaft is connected to suitable motor means mounted in the vibration housing 21 to impart rotation on the input shaft to rotate the outer drum 24 in a first direction. The axis of the input shaft 26 extends substantially through the center of gravity of the development housing 21 to reduce the aforementioned coupling effects. A gear means 27 including a spur gear 28 is secured at the end of the input shaft 26 to mesh with a second spur gear 29 supported by suitable means (not shown) on the housing 11 at a location adjacent the input shaft. Rotation of the spur gear 29 drives two pulleys 30 and 31 and a timing belt 32 to impart a rotation to drive shaft 33. The drive shaft 33 is rotatably mounted along the axis of the input shaft, but the two shafts 26 and 33 are rotated in opposite directions by the action of gearing means 27. A second inner drum 34 is attached to the end of the drive shaft to be rotated thereby and the outer surface thereof detachably supports the second weight 23 with a suitable bolt means 35 similar as utilized on the-outer drum. From the foregoing it should be apparent that the two eccentric weights-22 and 23 supported respectively by the two drums 24 and 34 are rotated about the same axis in opposite directions and in synchronization with each other.

The magnitudes of the two weights and the rate of rotation of input shaft 26 can be selected to be any value which imparts the desired level of oscillation of the developer bed. However, as described previously it is advantageous that the ratio of M R =M R be selected to produce a cancellation of the centrifugal force components of the system. The rotation of the weight assembly of Fig. 5 imparts a selected level of oscillation on the developer bed to fluidize the material thereon for development of the image on the drum without transmitting significant vibration to the xerographic machine. The amplitude of oscillation may be readily adjusted by varying either the magnitude of the weights or a radius of rotation thereof. Further, the frequency of oscillation is adjustable by varying the driving frequency of the drums. For example, the respective radius of rotation of the eccentric weights may be varied by the use of different diameter drums which allows for selective control of the amplitude of oscillation imparted to the bed. As the developer is fluidized within the bed ll, any suitable means (not shown) to add toner to the system as the powder is depleted during development may be utilized with the present invention. Also, the bed 11 may be mounted at different locations around the drum 1 than as illustrated in FIG. 3, if desired.

Referring now in particular to FIG. 6 there is illustrated still another embodiment of the development device 10 according to the present invention. The development device 10 includes a housing assembly 40 mounted on the shaft 41 supporting the photosensitive drum. The housing assembly 40 is mounted by suitable means on the shaft of the drum so that it swings in an arc in a manner similar to a pendulum. The bottom 42 of the housing assembly includes a bed 43 to support developer material in the vicinity of the drum surface to develop the latent electrostatic image lying thereon. By imparting a vibratory movement to the entire assembly of the housing 40, the developer material lying therein is fluidized to develop the latent image. The housing 40 may further include suitable vanes, wires, screens, and the like (not shown) to aid in agitating the developer into a fluidized state. The housing assembly simulating a pendulum is vibrated by the vibration inducing apparatus 20 comprising two counter rotating weights as described in reference with the embodiment of FIG. 3. However, in order to achieve advantageous development results, the vibration inducing apparatus 20 including the counter rotating weights 22 and 23 are mounted to orbit around an axis passing through the center of percussion of the housing assembly 40. By locating the rotating weights at the center of percussion of the housing assembly, it has been found that the forces transferred to the xerographic machine which would interfere with proper reproducing results are minimized.

The vibration inducing apparatus 20 may comprise any suitable mechanism capable of rotating the weights in an opposite direction around a common axis similar as described in reference to the embodiment of FIG. 3. The vibration inducing assembly 20 described in reference to FIG. 5 is one example of a suitable mechanism for imparting the desired vibration to the housing assembly 40 for fluidizing the developer. In FIG. 6 the axis around which masses such as eccentric weights 22 and 23 orbit is illustrated as being perpendicular to the axis of rotation of the drum 1, but it is within the scope of the present invention to orbit the weights around other extending axes depending on desired development results. Further, as mentioned previously, it is advantageous that the axis of rotation of the weights pass through the center of percussion of the housing assembly, but other locations may be selected if preferable. Also, the housing assembly 40 may be mounted on other axes other than the axis of rotation of the drum 1 to swing with a different are depending on encountered conditions. Therefore, the counter rotating weights apply a force to the housing assembly 40 which varies in magnitude and direction depending on the instantaneous position of the weights in their orbiting path to result in vibration of the housing and thus fluidization of the developer thereon. The masses of the weights, their radius of rotation, and the angular velocity of the vibration inducing apparatus may be varied to selectively control the degree of vibration in a manner similar as previously described in regard to the embodiment of FIG. 3. Accordingly, the embodiment of FIG. 6 provides a high degree of fluidization of the developer with selective control of oscillation of the housing and a minimum of vibration being transmitted to the machine support structure.

In the above description there has been disclosed an improved apparatus for effectively developing a latent electrostatic image supported on a xerographic photosensitive surface. The surface to be developed has been described for convenience of illustration as being that of a xerographic drum, but the invention may be used to develop other well-known photosensitive members in the form of plates, belts, webs or coated papers. it is further within the scope of the present invention to apply a vibratory force to produce oscillation of the housing to other housing assemblies other than the specific structure herein described. Further, for example, the housing assembly may encompass a bed or plate supporting developer material being cascaded thereon to create a fluidization of the flowing developer. Also, the housing may include the many advantageous features utilized in xerographic development as, for example, including an electrode surface to prevent toner deposition in nonimage areas of the drum and to increase solid image area development, means to tonerize the developer during development and the like. Also, the vibration inducing apparatus of the present invention may be utilized to vibrate any structure wherein a selectively controlled level of vibration is desired other than in xerographic development.

While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situa tion or material to the teaching of the invention without departing from its essential teachings.

What is claimed is: 1. An apparatus for developing a surface bearing a latent electrostatic image with developer material comprising housing means adapted to support developer material adjacent an image bearing surface to be developed,

oscillating means operatively connected to said housing means to vibrate said housing means for agitating developer material therein into development contact with the image bearing surface,

said oscillating means including weight means mounted for eccentric movement about an axis, and

motion inducing means coupled to said weight means to ef fect movement ofsaid weight means about said axis to impart a vibratory motion on said housing means.

2. The apparatus of claim I wherein said weight means includes first and second weights mounted for opposing eccentric movement about said axis.

3. The apparatus of claim 2 wherein said first weight is mounted at a first distance from said axis and said second weight is mounted at a second distance from said axis for opposing eccentric movement thereabout.

4. The apparatus of claim 3 wherein the arithmetical product of the magnitude of mass of said first weight and said first distance equals the arithmetical product ofthe magnitude of mass of said second weight and said second distance.

5. The apparatus of claim 4 wherein said first distance is unequal to said second distance.

6. The apparatus of claim 1 wherein said housing means is mounted on resilient means.

7. The apparatus of claim 6 wherein said axis passes through the center ofgravity ofsaid housing means.

8. The apparatus of claim 1 wherein said housing means is pivotally mounted to effect pendulous movement upon being vibrated by said oscillating means.

9. The apparatus ofclaim 8 wherein said axis passes through the center of percussion ofsaid housing means.

10. The apparatus of claim I wherein said houslng means supports developer material comprising carrier particles and electroscopic toner powder and said oscillation means agitates the developer material into a fluidized state.

11. In an apparatus for developing a surface bearing a latent electrostatic image in which a developer supporting housing containing a quantity of developer material is mounted adjacent the surface to be developed and is vibrated to agitate the developer material into contact with the surface to be developed, the improvement comprising oscillating means including counter rotating weight means for vibrating said housing to agitate the developer material into contact with the surface to be developed 12. The apparatus of claim 11 wherein said weight means comprises a pair of eccentrically mounted weights rotating in opposite directions around a common axis.

13. The apparatus of claim 12 wherein said axis passes through the center of gravity ofthe housing means.

14. The apparatus of claim 12 wherein said housing is pivotally mounted and said axis passes through the center of percussion thereof.

PO-lOfiU (5/65) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent: No. 3 ,623 IS I Dated January 26 1972 Inventor(s) John S. Bernhard et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE SPECIFICATION:

Column 1, line 6%, "NOrmally" should be --Normally-.

"of" should be --or--.

Column 3 line 51,

Column I, lines 35-37 "After development, material comprising carrier particles and electrosoopic toner for development thereof. should be deleted in its entirety.

Column t, line H6 after the word transfer" --devioe I-- should be inserted.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer ROBERT GOTTSCHALK Commissioner of Patents 

2. The apparatus of claim 1 wherein said weight means includes first and second weights mounted for opposing eccentric movement about said axis.
 3. The apparatus of claim 2 wherein said first weight is mounted at a first distance from said axis and said second weight is mounted at a second distance from said axis for opposing eccentric movement thereabout.
 4. The apparatus of claim 3 wherein the arithmetical product of the magnitude of mass of said first weight and said first distance equals the arithmetical product of the magnitude of mass of said second weight and said second distance.
 5. The apparatus of claim 4 wherein said first distance is unequal to said second distance.
 6. The apparatus of claim 1 wherein said housing means is mounted on resilient means.
 7. The apparatus of claim 6 wherein said axis passes through the center of gravity of said housing means.
 8. The apparatus of claim 1 wherein said housing means is pivotally mounted to effect pendulous movement upon being vibrated by said oscillating means.
 9. The apparatus of claim 8 wherein said axis passes through the center of percussion of said housing means.
 10. The apparatus of claim 1 wherein said housing means supports developer material comprising carrier particles and electroscopic toner powder and said oscillation means agitates the developer material into a fluidized state.
 11. In an apparatus for developing a surface bearing a latent electrostatic image in which a developer supporting housing containing a quantity of developer material is mounted adjacent the surface to be developed and is vibrated to agitate the developer material into contact with the surface to be developed, the improvement comprising oscillating means including counter rotating weight means for vibrating said housing to agitate the developer material into contact with the surface to be developed.
 12. The apparatus of claim 11 wherein said weight means comprises a pair of eccentrically mounted weights rotating in opposite directions around a common axis.
 13. The apparatus of claim 12 wherein said axis passes through the center of gravity of the housing means.
 14. The apparatus of claim 12 wherein said housing is pivotally mounted and said axis passes through the center of percussion thereof. 